An accessory allowing two Game Boy units to connect and communicate with each other. The original Game Boy included the link cable to facilitate head-to-head games of Tetris, but units since then have sold the cable separately.
The Game Boy Pocket, Game Boy Color, and Game Boy Advance have a different connector shape than the original Game Boy, although adapters are available to map the pins. The Game Boy Advance has one extra pin, and cables designed for it must be plugged in appropriately. The larger connector on the GBA cable is for Player 1, while the smaller connector is for Player 2. Some Game Boy Advance games support single-pak multiplayer, where player 2 powers on with no Game Pak inserted and Player 1 uploads a subsection of the game for play.
Two efforts have been made to connect multiple Game Boys using the link cable. F-1 Race for the original Game Boy came with a 4-player adapter: player 1 plugged the adapter directly into his Game Boy, while the other three players connected their Game Boys to the adapter. The only other game of note to use this adapter was FaceBall 2000. If you get the official Nintendo Game Boy Advance cable, it includes two connectors with a small one-port hub in the center. To connect two Game Boys in this manner, you ignore the hub and connect them directly; for three Game Boys, connect two directly and a third to the hub; for four Game Boys, connect two directly, one to the first hub, and a second to Player 3's hub. The official instruction manual has a diagram illustrating this setup, which looks fairly complicated. Potentially, you could connect an infinite number of machines to each other using cables, but games only support 4 players. (The Atari Lynx allowed you to connect 8 players in series using the ComLynx cable.)
It is believed that Pokemon, which requires linking with different versions of the games, was a major catalyst to the sales of link cables. A special Pikachu link cable was made and sold in Japan; it features a small Pikachu figure which appears to be looking down at your game as you play. Very creepy and cute at the same time.